You must not be from Europe. If you were familiar with vampire lore, you wouldn’t have said that. No offense!🙃I didn't notice it until you mentioned it but it is a weird line coming from a non-Japanese vampire that is hundreds of years old. Sometimes you can feel the mangaka behind the words more than the character and this is one of those cases.
According to old vampire lore, the younger (not a kid, ofc), purer, and untouched a girl was, the better her blood was considered. That’s why, by their standards, even a 25-year-old would already be seen as old, since there was a high chance she had already been ‘tainted’ (in the 17-18.th century, this would have been very likely) So, actually the manga follows this legend closely.